Unavoidable concrete segregation during concrete pouring can lead to cold joint problems such as water leakage at wall to slab/footing junctions.
What causes concrete segregation?
Over the past 12 months, the Dincel Group has conducted extensive AS3735 waterproofness tests with excellent results. These tests have revealed that irrespective of using self-compacting concrete (SCC) or conventional concrete in permanent formwork systems*, besser blocks or conventional steel/plywood formwork, segregation is largely unavoidable under any circumstance.
With conventional formwork, segregation is usually associated with the free fall of aggregates from height. In the case of permanent polymer formwork systems, the internal webs have perforations that work like an elephant’s trunk, which stops the free fall of aggregates, thereby, eliminating segregation.
Observations by Dincel have, however, revealed that there is another reason for concrete segregation: the concrete sitting within the pump hose before the commencement of concrete pouring.
The concrete waiting within the pump hose loses cement paste, leaving greater accumulation of coarse aggregate. The longer the wait time to start concrete placement, the worse is the segregation of aggregate from the cement paste.
Practically, it is not possible to disperse and mix the already segregated concrete at the bottom of the wall – even with good vibration practices. The reality is that even expert vibration practices will only begin after the placement of at least a few hundred millimetres of concrete depth.
There is a solution
The only way to avoid this problem is to discharge the already segregated concrete before beginning with the placement of concrete. Only a minimal amount of concrete is needed to be discharged to waste, but this will ensure a cohesive wall to slab/footing junction.
The concrete waiting within the pump hose at the kink created by the hose clamp loses cement paste, leaving greater accumulation of coarse aggregate.
Prior to starting concrete placement, discharge the segregated concrete.
Concrete sitting within the pump hose is discharged into a barrel prior to the commencement of the concrete pour.
* polymer, fibre-cement, double-precast wall
